Optimize topology_span_sane() by removing duplicate comparisons.

Since topology_span_sane() is called inside of for_each_cpu(), each
pervious CPU has already been compared against every other CPU. The
current CPU only needs to be compared against higher-numbered CPUs.

The total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 on each non-NUMA scheduling domain level.

Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/topology.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 99ea5986038c..b6bcafc09969 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2347,7 +2347,7 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
 static bool topology_span_sane(struct sched_domain_topology_level *tl,
 			      const struct cpumask *cpu_map, int cpu)
 {
-	int i;
+	int i = cpu + 1;
 
 	/* NUMA levels are allowed to overlap */
 	if (tl->flags & SDTL_OVERLAP)
@@ -2359,9 +2359,7 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
 	 * breaking the sched_group lists - i.e. a later get_group() pass
 	 * breaks the linking done for an earlier span.
 	 */
-	for_each_cpu(i, cpu_map) {
-		if (i == cpu)
-			continue;
+	for_each_cpu_from(i, cpu_map) {
 		/*
 		 * We should 'and' all those masks with 'cpu_map' to exactly
 		 * match the topology we're about to build, but that can only
-- 
2.44.0

On 10/04/24 16:33, Kyle Meyer wrote:
> Optimize topology_span_sane() by removing duplicate comparisons.
>
> Since topology_span_sane() is called inside of for_each_cpu(), each
> pervious CPU has already been compared against every other CPU. The
> current CPU only needs to be compared against higher-numbered CPUs.
>
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 on each non-NUMA scheduling domain level.
>
> Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
> Reviewed-by: Yury Norov <yury.norov@gmail.com>
> Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:
> Optimize topology_span_sane() by removing duplicate comparisons.
> 
> Since topology_span_sane() is called inside of for_each_cpu(), each
> pervious CPU has already been compared against every other CPU. The

previous

> current CPU only needs to be compared against higher-numbered CPUs.
> 
> The total number of comparisons is reduced from N * (N - 1) to
> N * (N - 1) / 2 on each non-NUMA scheduling domain level.

Thank you, now it makes sense.

-- 
With Best Regards,
Andy Shevchenko

On Thu, Apr 11, 2024 at 01:38:46PM +0300, Andy Shevchenko wrote:
> On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:
> > Optimize topology_span_sane() by removing duplicate comparisons.
> > 
> > Since topology_span_sane() is called inside of for_each_cpu(), each
> > pervious CPU has already been compared against every other CPU. The
> 
> previous

Thank you for pointing that out. Should I send an updated version or can
a maintainer correct my mistake?

> > current CPU only needs to be compared against higher-numbered CPUs.
> > 
> > The total number of comparisons is reduced from N * (N - 1) to
> > N * (N - 1) / 2 on each non-NUMA scheduling domain level.
> 
> Thank you, now it makes sense.

Thanks,
Kyle Meyer

On Thu, Apr 11, 2024 at 04:55:11PM -0500, Kyle Meyer wrote:
> On Thu, Apr 11, 2024 at 01:38:46PM +0300, Andy Shevchenko wrote:
> > On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:

...

> > > Since topology_span_sane() is called inside of for_each_cpu(), each
> > > pervious CPU has already been compared against every other CPU. The
> > 
> > previous
> 
> Thank you for pointing that out. Should I send an updated version or can
> a maintainer correct my mistake?

Depends on the maintainer. I'm not the one here, don't expect answer from me.

> > > current CPU only needs to be compared against higher-numbered CPUs.

-- 
With Best Regards,
Andy Shevchenko

On Fri, Apr 12, 2024 at 05:25:26PM +0300, Andy Shevchenko wrote:
> On Thu, Apr 11, 2024 at 04:55:11PM -0500, Kyle Meyer wrote:
> > On Thu, Apr 11, 2024 at 01:38:46PM +0300, Andy Shevchenko wrote:
> > > On Wed, Apr 10, 2024 at 04:33:11PM -0500, Kyle Meyer wrote:
> 
> ...
> 
> > > > Since topology_span_sane() is called inside of for_each_cpu(), each
> > > > pervious CPU has already been compared against every other CPU. The
> > > 
> > > previous
> > 
> > Thank you for pointing that out. Should I send an updated version or can
> > a maintainer correct my mistake?
> 
> Depends on the maintainer. I'm not the one here, don't expect answer from me.

I like this rework, and I'll take it in bitmap-for-next for testing.

I see Vincent already acked it, so I can also move it upstream, if no
objections from Peter, Juri and Ingo.

Thanks,
Yury